LeetCode - Binary Tree Zigzag Level Order Traversal

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).

For example:
Given binary tree {3,9,20,#,#,15,7},

    3

   / \

  9  20

    /  \

   15   7

 

return its zigzag level order traversal as:

[

  [3],

  [20,9],

  [15,7]

]

Solution:

 1 /**

 2  * Definition for binary tree

 3  * public class TreeNode {

 4  *     int val;

 5  *     TreeNode left;

 6  *     TreeNode right;

 7  *     TreeNode(int x) { val = x; }

 8  * }

 9  */

10 public class Solution {

11     public ArrayList<ArrayList<Integer>> zigzagLevelOrder(TreeNode root) {

12         // Start typing your Java solution below

13         // DO NOT write main() function

14         ArrayList<ArrayList<Integer>> ans = new ArrayList<ArrayList<Integer>>();

15         LinkedList<TreeNode> currentLevel = new LinkedList<TreeNode>();

16         int level = 0;

17         if(root != null) 

18             currentLevel.add(root);

19         while(!currentLevel.isEmpty()){

20             LinkedList<TreeNode> nextLevel = new LinkedList<TreeNode>();

21             ArrayList<Integer> list = new ArrayList<Integer>();

22             for(TreeNode node : currentLevel){

23                 list.add(node.val);

24                 if(node.left != null)

25                     nextLevel.add(node.left);

26                 if(node.right != null)

27                     nextLevel.add(node.right);

28             }

29             if(level%2 == 1)

30                 Collections.reverse(list);

31             ans.add(list);

32             currentLevel = nextLevel;

33             level++;

34         }

35         return ans;

36     }

37 }